Insufficient dehumidification in indoor facilities can lead to the deterioration of building materials as well as cause serious moisture-related health issues. Conventional HVAC systems utilize mechanical refrigeration to achieve both sensible and latent cooling. Latent cooling (i.e., dehumidification) occurs when the air is passed over a cooling coil, thereby lowering the air temperature below the entering dew point and causing a portion of the moisture in the air to condense on the coil's surface and drop out of the airflow.
Mechanical refrigerative dehumidification is most effective when the air is at higher temperatures and the relative humidity approaches 100%. However, the latent cooling efficiency of mechanical refrigeration diminishes in low relative humidity environments. In such dry conditions, the air temperature must be cooled below the entering dew point in order to remove moisture from the air. The resulting cold air then must be reheated to avoid over-cooling the space, thereby increasing energy use. Additionally, in subfreezing dew point applications such as ice rink arenas, periodic defrosting cycles are necessary due to ice accumulation on the cooling coils.
Desiccant wheels have been incorporated into air handling systems to replace or enhance the dehumidification performance of mechanical refrigerative dehumidification. Unlike mechanical refrigeration which relies upon cooling the air below its dew point, desiccant dehumidification relies on adsorption. Moisture transfer by the desiccant is driven by the difference in relative humidity of the “process” and “regeneration” air streams. When the relative humidity of the regeneration air stream is lower than the relative humidity of the process air stream, the desiccant will adsorb moisture from the process air stream and transfer it to the regeneration air stream.
Desiccant dehumidification systems are typically designed in either dual-path or single-path configurations depending on the application. A schematic illustration of a conventional dual-path desiccant dehumidification system is shown in FIG. 1. In the dual-path configuration, two counter-current air streams power the operation of the desiccant dehumidification system. The desiccant wheel rotates through these two air streams and transfers moisture from the higher relative humidity process air stream to the lower relative humidity regeneration air stream. Because the relative humidity of the air leaving the process side of the wheel can only get as low as the relative humidity of the air entering the regeneration side of the wheel, a heat source is typically utilized to heat the regeneration air stream to lower its relative humidity. Typical heat sources include direct-fired gas heaters, electric heaters, and indirect heat sources such as steam, hot water, solar, and waste heat from the building. Depending on the targeted indoor conditions, regeneration air temperatures usually range from 100° F. to 300° F., which in turn raises the dry-bulb temperature of the process air leaving the desiccant wheel. Accordingly, most dual-path systems include a cooling coil downstream of the process side of the wheel to re-cool the air before supplying it to the conditioned space.
A schematic illustration of a conventional single-path desiccant dehumidification system is shown in FIG. 2. These systems are sometimes referred to as “Cromer Cycle” systems. Single-path desiccant dehumidification systems are designed to enhance the dehumidification performance of a traditional cooling coil in applications which would otherwise be difficult and expensive to maintain using mechanical refrigeration alone. In such systems, moisture transfer occurs within a single air stream. The desiccant wheel is configured in series with a cooling coil such that the regeneration side of the wheel is located upstream of the coil and the process side of the wheel is located downstream of the coil. The air downstream of the cooling coil will be at a very high relative humidity as it enters the process side of the desiccant wheel. The desiccant wheel will adsorb moisture from the saturated air downstream of the coil and deposit it back into the air upstream of the coil. This moisture will then be removed from the air by the coil via condensation. The addition of the desiccant wheel to the conventional mechanical refrigeration system enhances the dehumidification performance of the traditional cooling coil by increasing the latent capacity of the cooling coil without increasing its total cooling capacity. And, unlike a conventional mechanical refrigeration system with a cooling coil alone, the supply air dew point can be lower than the coil surface temperature.
In certain applications, single-path desiccant dehumidification systems are capable of providing significant energy savings over dual-path desiccant dehumidification systems. Unlike dual-path systems, single-path systems typically do not require an external heat source to regenerate the desiccant wheel. Further, post-cooling may not be necessary with single-path systems, whereas the process air stream in dual-path systems oftentimes must be re-cooled before it's supplied to the conditioned space.
A shortcoming of current-generation single-path desiccant systems, however, is the inability to drastically reduce moisture content from the processed air. Further, the effectiveness of current generation single-path systems is significantly diminished in applications where the air entering the system has a high relative humidity. This is only exacerbated where the incoming air has a low temperature in addition to high relative humidity. In such conditions, periodic defrosting cycles may be necessary due to frost buildup on the coils. As a result, dual-path systems are still predominately used in low dew point applications such as ice rink arenas despite their high energy usage per pound of water removed.